A serum-free defined media system was developed for the BALB/MK mouse kertinocyte cell line. It was found that this cell line required only two growth factors, insulin and epidermal growth factor (EGF), for growth rates which approached those achievable in serum-containing media supplemented with EGF. This defined media system was then used to determine the precise growth factor requirements of BALB/MK cells transformed with a variety of oncoviruses. It was found that nearly all of the viral transformants were able to grow in the absence of EGF. However, different oncoviruses varied in how efficiently they abrogated EGF dependence. H-ras-, K-ras-, fgr- and abl-transformed cells became entirely EGF-independent. Fms-, mos-, and erbB-transformed cells grew in the absence of EGF, but grew better when EGF was present. Cells transformed with fgr and mos were also able to grow in the absence of EGF. This defined media system has also been adapted as a method of stringently selecting for cells which escape from growth factor requirements following transformation.